Closed loop operation of induction motor drive systems without shaft transducers is commonly referred to as sensorless, tachless, or transducerless. Eliminating the need for rotor shaft transducers such as encoders, resolvers, and tachometers for position and velocity estimations in an induction motor drive system can reduce system cost, improve system reliability, and reduce the total motor package size.
Several approaches for providing sensorless control of induction motors are described in U.S. Pat. Nos. 5,565,752 and 5,585,709 wherein a determinist spatial modulation in the rotor leakage inductance of the induction motor is tracked via the demodulation of an injected AC signal voltage or current. Tracking of the modulated rotor leakage inductance provides an estimate of the rotor position and velocity that can then be used for sensorless control.
U.S. Pat. No. 5,565,752 describes induction motor rotors with spatial variations in the rotor leakage inductance to enable rotor position and velocity tracking. The patent describes special rotor slot designs that have either open slots or a combination of open and closed slots. Although large motors often have bar wound rotors with open slots, the majority of conventional small induction motors, however, are fabricated with closed rotor slots with the rotor bars being cost-effectively formed by casting molten aluminum (these rotors are usually machined and sometimes the bridges are removed and the slots are opened after casting). U.S. Pat. No. 5,565,752 does not disclose manufacturing methods or rotor designs that are readily manufacturable with existing industrial casting processes and equipment.
It is therefore seen to be desirable to provide a cost effective method of creating spatial modulation. The present invention provides rotor lamination fabrication techniques to create the desired spatial modulation in the rotor leakage inductance and/or the rotor resistance which can be used with AC signal injection to track rotor positions and velocities.
Briefly, in accordance with one embodiment of the present invention, a method for fabricating an induction motor rotor lamination comprises using an indexed notching die to punch a plurality of closed rotor slots at different distances from a center point of rotation by first punching a first rotor slot, changing the position of a center point of the rotor lamination with respect to the notching die, and then punching a second rotor slot.
Another method for fabricating an induction motor rotor lamination comprises: punching a plurality of closed rotor slots and rotor slits in a lamination; and using an indexed notching die having a wider rotor slit to selectively punch and widen predetermined ones of the rotor slits.
Another method for fabricating an induction motor rotor lamination comprises: punching a plurality of rotor slit patterns using a variable depth indexed notching die including at least two rotor slit punching portions, one of the rotor slit punching portions being deeper than another of the rotor slit punching portions, by punching a first rotor slit pattern, changing the depth of closure of the notching die, and punching a second rotor slit pattern, the second rotor slit pattern having a different number of rotor slits than the first rotor slit pattern.
Another method for fabricating an induction motor rotor comprises: providing a plurality of rotor laminations each including a plurality of rotor slots situated at different distances from a center point of rotation and a plurality of rotor slits, each rotor slit being open to and off center with respect to its respective rotor slot; stacking the plurality of rotor laminations by positioning some of the rotor laminations in an orientation which is turned over and rotated with respect to others of the rotor laminations; filling the rotor slots and the rotor slits with cage material; and machining outer surfaces of the rotor laminations.
In another embodiment of the present invention, uniformly spaced identical rotor slots are punched and modulation is created by punching a contoured outside diameter or by inserting slot wedges of molded magnetic material.